Digital signals can be transmitted by frequency-shift keying (FSK) modulation. Frequency-shift keying modulation comprises shifting a continuous running carrier in frequency between two closely spaced frequencies according to the logical highs and lows being transmitted. The use of frequency-shift keying modulation can be effective in the presence of large signal fading from changing propagation conditions.
Some conventional demodulation circuits for FSK modulated signals use a differential amplifier to compare the outputs from a pair of filters set at the two frequencies corresponding to logical high and low. Narrow shift FSK modulated signals have been used to circumvent selective fading between the two signal frequencies. However, the shift cannot be reduced below the information band width of the keyed signal itself.
One application of a high frequency FSK modulated signal is for communication in a contactless key identification system. In such a system, a transponder in a key transmits a signal to a key reader associated with a key lock. One problem that can occur with FSK modulated signals transmitted by a transponder in a key is a drift in frequency due to ambient temperature and other conditions. A drift in frequency causes a corresponding drift in the frequencies corresponding to logical high and low. This causes problems with processing the FSK modulated signal to detect transmitted data.